Spell Catcher is the modern-day descendant of a spell-checking app from ancient times (as a Desk Accessory called Thunder, from 1985), now with powerful, system-wide spell-checking and text-expanding features.
By default, this muscular app tracks every word you type on your Mac, across every application, and provides you with much more than the live visual feedback you get in separate apps with built-in spell-checking. Spell Catcher can give you automatic typo fixing, spoken alerts (for example, pointing out repeated words, or curious spellings with a quick "Curious!"), instant access to the dictionary and thesaurus (with support for multiple dictionaries and online sources), and serious text-expansion capabilities that let you set up an extensive "shorthand glossary" for frequently used words, phrases, or even multipage documents. Spell Catcher also includes a "Ghostwriter" feature, which can be a life-saver in the event of a crashed app or lost doc: it records all your keystrokes and saves those to a text file, which you can recover later.
For the most part, Spell Catcher runs invisibly in the background, but you can invoke its features directly through the app, using keyboard shortcuts, or through the Dock, Input, or Services menus. The initial setup for Spell Catcher can be time-consuming, getting its settings to match your preferred work flow and habits, but this app has excellent documentation, active forums, and a responsive developer. Note: now that Spell Catcher has been fully updated for Snow Leopard, version 10.3.4 is the last to support Mac OS 10.4 (Tiger).